1. Field of the Invention
The present invention relates generally to methods for removing a film formed on a surface of a silicon wafer or the like, and particularly it relates to a method for removing a film such as a native oxide film formed on a surface of a silicon wafer, a surface of a polysilicon film, or a surface of an amorphous silicon film, etc. (such surface being hereinafter referred to as "silicon layer surface") by using halide such as hydrogen fluoride (HF).
2. Description of the Related Art
In a process of manufacturing semiconductor devices, semiconductor substrates are liable to be contaminated by various factors. Such contamination adversely affects the operating characteristics of the semiconductor devices to be formed thereafter on the substrates. Such contamination decreases the yield of production of semiconductor devices. Therefore various efforts have been made to protect the silicon layer surfaces of wafers from contamination in respective steps of the manufacturing process.
A native oxide film (SiO.sub.2) formed on a silicon layer surface is considered to be one of contaminants on the silicon layer surface. The native oxide film is easily formed to a thickness of 10 to 20 .ANG. on the silicon layer surface by only exposing the surface to the atmosphere. Native oxide films are also formed secondarily on silicon layer surfaces in different cleaning and etching steps in a semiconductor device manufacturing process.
For instance, it is known that electric characteristics of a thin gate oxide film of a MOSFET (Metal-Oxide Silicon Field Effect Transistor) formed on a silicon wafer are considerably affected by preliminary treatment of the silicon wafer surface. If a thin oxide film such as a gate oxide film is to be formed on a silicon wafer in a semiconductor device manufacturing process, it is necessary to remove a native oxide film in advance. Otherwise, the yield of production would be lowered.
The native oxide film not only causes the electric characteristics of the thin gate oxide film to be unstable but also exerts adverse effects as described below. In the case of a MOSFET, if a native oxide film remains on a base layer where electrodes such as a source and a drain are to be formed, the electrodes formed thereon can not perform their functions normally. If a metal electrode is formed on the silicon wafer where a native oxide film exists, contact resistance increases due to the existence of the native oxide film. In the case where a silicon crystal layer is to be formed on a substrate by epitaxial growth, the crystal layer having desired characteristics could not be obtained if a native silicon oxide film exists on the substrate surface.
Thus, special care should be taken for the native oxide film formed on the silicon wafer surface in the semiconductor device manufacturing processes. Particularly, before formation of a film by CVD (Chemical Vapor Deposition), or sputtering or the like, it is always necessary to remove the native oxide film from the wafer surface.
Various methods have been proposed and used to remove the native oxide film from the silicon wafer surface. In recent years, several methods for cleaning silicon wafer surfaces by using halide gas such as hydrogen fluoride (HF) have been studied.
One example of such methods is disclosed in "Submicron ULSI Process Technology" in papers prepared for the seventh ULSI Ultra Clean Technology Symposium, pp. 173-174 issued on July 1988 by Realize Co. Ltd. According to this method, anhydrous hydrogen fluoride gas having a very low content of water is sent into a reaction chamber using nitrogen or argon gas as a carrier gas. The hydrogen fluoride gas introduced into the reaction chamber reacts with a silicon oxide film (SiO.sub.2) on a silicon wafer in the chamber, whereby the silicon oxide film is removed.
A second example is disclosed in U.S. Pat. No. 4,749,440. According to the method disclosed therein, anhydrous hydrogen fluoride gas is supplied to a silicon wafer surface together with water vapor. In the same manner as in the above-described method, the hydrogen fluoride gas reacts with the silicon oxide film on the silicon wafer surface, whereby the silicon oxide film is removed.
In the above-described examples, the native oxide film of silicon is mentioned as one of contaminants. However, the contaminants on the wafer are not limited to the native oxide film of silicon. The hydrogen fluoride used in the above-mentioned two methods also acts as a contaminant of the wafer if it remains thereon.
In a semiconductor device manufacturing process, halide such as nitrogen fluoride (NF.sub.3), sulfur hexafluoride (SF.sub.6), chlorine trifluoride (ClF.sub.3) or chlorine (Cl.sub.2) other than hydrogen fluoride is often used. The halide is used not only in the case of removing the oxide film from the wafer surface as described above but also in the case of etching a metal film such as an aluminum film formed on the wafer surface, or the surface layer itself of the wafer of another kind.
Any of the above mentioned halide is used in an active state of ions, radicals or the like. In a stage where an etching or cleaning process is terminated, ions of halogen such as fluorine or chlorine remain on the substrate surface and contaminate it.
If a device is formed on the substrate surface contaminated by the halogen ions, various defects of the device are liable to occur. For instance, a film is not formed in good condition, or the device obtained does not operate in an electrically correct manner.
Therefore, it is necessary to remove the ions remaining on the substrate surface after the etching or cleaning process. A conventional common method for removing the ionic contamination from the substrate surface is to clean the wafer surface by using high purity water (or ultrahigh purity water). Another method is disclosed in Japanese Patent Laying-Open No. 62-173720.
According to the above-mentioned laid-open application, vapor of hydrofluoric acid is supplied into a container in which a wafer is secured, so that a native oxide film on the wafer surface is removed. The hydrofluoric acid is removed from the wafer surface by vapor of high purity water also supplied into the container.
Other methods include a method for removing the ionic contamination from the substrate surface by exposing the wafer surface to ultraviolet light or applying argon ion (Ar.sup.+) sputtering to the substrate surface.
Thus, various methods for protecting wafer surfaces from contamination have been studied. However, it seems difficult to say that the wafer surface can be sufficiently protected from contamination in the prior art. The conventional method for removing contamination due to formation of films or the like, from the substrate surface, have various disadvantages as described below.
Anhydrous hydrogen fluoride gas exists as an ensemble by association of two to six molecules particularly at a temperature of 80.degree. C. or less. For instance, two molecules form an association (which is called a dimer). Consequently, reactivity between anhydrous hydrogen fluoride gas and SiO.sub.2 is very low. However, if water exists together, the hydrogen fluoride is dissociated to generate fluorine ions F.sup.-. The generated F.sup.- ions react on the silicon oxide film and etching reaction of SiO.sub.2 proceeds.
Thus, water plays an important role in the etching reaction of SiO.sub.2 with the hydrogen fluoride.
However, it is technically difficult to control the etching reaction of the silicon oxide film by the anhydrous hydrogen fluoride in an atmosphere having a very low content of water as described above.
For instance, hydrogen fluoride and silicon dioxide react according to the following formula. EQU 4HF+SiO.sub.2 .fwdarw.SiF.sub.4 +2H.sub.2 O
As is evident from the above formula, a large quantity of water is generated as a by-product as a result of the reaction. The generated water is liable to adhere to a reaction tube or a wall in the reaction chamber. In order to remove the water on the wall or the like, it is necessary to purge the reaction tube and the reaction chamber for long hours by using nitrogen gas of high purity.
Accordingly, in the case where the silicon oxide film is to be removed by using anhydrous hydrogen fluoride gas in an atmosphere having a very little quantity of water, it is difficult to control the reaction and to apply the process reliably with good repeatability.
The method described in the above-mentioned paper "Submicron ULSI Process Technology" has the following problems. This method requires a diluted gas of anhydrous hydrogen fluoride containing water of only about 0.01 ppm. It takes much time and labor to reduce the water content in the gas to such a small quantity.
On the other hand, according to the method of removing the silicon oxide film by adding water vapor to anhydrous hydrogen fluoride gas, a side reaction indicated in the following formula (1) proceeds. EQU SiO.sub.2 +H.sub.2 O.fwdarw.H.sub.2 SiO.sub.3 . . . (1)
Otherwise, a reaction opposite to the etching occurs as shown in the following formula (2). EQU 2SiF.sub.4 +2H.sub.2 O.fwdarw.SiO.sub.2 +SiF.sub.6.sup.2- +2H.sup.+ +2HF . . . (2)
As a result of those undesired reactions, colloidal metasilicic acid H.sub.2 SiO.sub.3 or silicon dioxide SiO.sub.2 adheres to the wafer surface and remains thereon. The wafer surface is newly contaminated by particles of the above-mentioned substances, causing the yield of production of wafers to be lowered.
The above-mentioned methods has other problems as described below. In a semiconductor manufacturing process, steps of forming various films are performed on the silicon wafer surface. Not only a native silicon oxide film but also an insulating film such as a thermal silicon oxide film or a silicon nitride film, and so on are formed on the silicon wafer. In the above-mentioned method, even when only the native silicon oxide film is to be removed, other insulating films or the like intentionally formed on the wafer are unavoidably removed together with the native oxide film. If it is possible to selectively remove the native silicon oxide film, the semiconductor manufacturing process will be considerably simplified.
The conventional methods also involve disadvantages as described below after removal of a contaminant film from the wafer surface. As described previously, ionic contamination remains on the wafer surface. Conventionally, this contamination is commonly removed by high purity water. However, according to the above-mentioned conventional methods, if silicon Si, metal such as aluminum or the like is exposed on the substrate surface, an oxide film is liable to be formed secondarily. The oxide film so formed on the wafer surface will have to be removed by another method.
If the wafer surface is cleaned by exposure to ultraviolet light or Ar ion sputtering, the silicon wafer surface is unavoidably damaged. As a result, defects are liable to occur in the device to be formed afterwards on the wafer surface.
Thus, in the prior art, it is not possible to remove an undesired film on the silicon wafer surface with good repeatability by using halide such as anhydrous hydrogen fluoride. In addition, it is not possible to selectively remove the native oxide film, and the wafer surface after treatment can not be effectively protected from contamination.